The efficiency of a refrigeration or heat pump system depends, to a large extent, on the amount of frost present on the evaporator coils thereof. Frost present on an evaporator coil tends to act as an insulator, and inhibits heat transfer between the evaporator coil and the atmosphere. Frost accumulation beyond a predetermined level can abruptly and drastically reduce the efficiency of the system.
Systems to control the amount of frost permitted to accumulate on the evaporator coils are known. Ideally, such systems prevent frost from accumulating beyond the predetermined level. However, in order to optimize efficiency of the system, it is desirable to maintain the frequency of the defrosting operation at the minimum necessary to prevent frost buildup beyond the critical limit.
Early systems employed mechanical cyclic timers which incorporated synchronous motors and complex gearing schemes to periodically actuate a fixed time defrost operation. Such early systems initiated defrost cycles without regard to changes in environmental conditions or to whether frost was actually present on a coil. This often resulted in unnecessary defrost cycling, the effect of which was to negatively impact the efficiency of the system. Examples of such systems are described in U.S. Pat. No. 3,277,662 issued Oct. 11, 1966 to T. B. Winters, and U.S. Pat. No. 3,541,806 issued Nov. 24, 1970 to H. W. Jocabs.
Later systems employing such timer motors included provisions intended to mitigate the problem of unnecessary defrost operations. For example, various systems inhibited (suspended) operation of the timer during periods when atmospheric conditions surrounding the evaporator coil were not contributory to a defrosting requirement. Examples of such systems are described in U.S. Pat. No. Re 3,164,969 issued to M. Baker on Jan. 12, 1965, and U.S. Pat. No. 26,596 reissued Jun. 3, 1969 to H. W. Jobes. Other systems employing similar timer motors, such as that described in U.S. Pat. No. 4,358,933 issued Nov. 16, 1982 to J. B. Horvay, advance the motors only during periods when the compressor is running, or as in those systems described in U.S. Pat. No. 4,344,294 issued Aug. 17, 1982 to R. B. Gelbard, and U.S. Pat. No. 4,056,948 issued Nov. 8, 1977 to C. J. Goodhouse, which vary the frequency of the motors in accordance with a sensed parameter, e.g., temperature or humidity.
Other examples of defrost control systems are described in U.S. Pat. No. 4,481,785, issued to Tershak, et al., on Nov. 13, 1984; 4,251,999 issued Feb. 24, 1981 to Y. Tanaka; 3,312,080 issued Apr. 4, 1967 to J. A. Dahlgran; 3,399,541 issued Sept. 3, 1968 to R. Thorner; 4,297,852 issued Nov. 3, 1981 to R. B. Brooks and 3,727,419 issued Apr. 17, 1973 to Brightman, et al.
Adaptive defrost control systems are also, in general, known. For example, such an adaptive system is described in U.S. Pat. No. 4,251,988, issued to Allard and Heinzen on Feb. 24, 1981, and is commonly assigned with the present invention. The Allard and Heinzen defrosting system automatically seeks to defrost a heat transfer unit such as an evaporator coil when the critical limit of frost has accumulated. The time required to actually defrost the coil is monitored, and the time period between defrosting operations is adjusted until no more than the critical amount of frost builds up on the coil before the next defrosting operation is initiated. More specifically, the Allard and Heinzen system monitors the actual defrost time of the evaporator coil for each defrost operation. An actual defrost time shorter than a predetermined optimal time indicates that less than the optimal amount of frost was allowed to accumulate and, concomitantly, that the defrosting operation is being performed more frequently than necessary. Accordingly, the system lengthens the time between successive defrost periods. An actual defrost time longer than the predetermined optimal time indicates that too much frost was allowed to accumulate. The system, therefore, shortens the time period between successive defrost operations. To this end, the Allard, et al, system operates according to the following relationship: EQU Ta=T.sub.(a-1) +K(Dd-Da),
wherein
Ta=Length of the next frost accumulating period. PA0 T.sub.(a-1) =Length of the last frost accumulating period. PA0 Dd=Desired (optimal) defrost time period. PA0 Da=Length of the actual defrost period. PA0 K=System constant that determines the multiple by which the frost accumulating period will change for each minute of error in the defrost time.
In the context of systems such as a heat pump system, where the evaporator coil is relatively exposed to the elements, the known adaptive defrost systems may be susceptible to inefficiencies due to changes in the frost buildup characteristics caused by abrupt changes in environmental conditions. For example, exposure of the evaporator coil to weather phenomena such as fog, freezing rain, sleet, or snow can dramatically alter the rate of frost buildup. In some instances, changing environmental conditions may cause frost buildup to exceed a predetermined optimum level far in advance of the next defrost operation, particularly in those instances when the defrost cycle is scheduled based upon the previously measured defrost time. Likewise, changes in ambient temperature may vary the time period required to defrost the coils independently of the amount of actual frost buildup, thus interjecting an indefiniteness into the defrost period measurement.
Defrost controllers responsive to changes in ambient conditions are also known. For example, U.S. Pat. No. 4,573,326, issued Mar. 4, 1986 to Sulfstede, et al, describes an adaptive defrost control for a heat pump system wherein a defrost cycle is initiated when the difference between the ambient temperature and the temperature of the heat exchange unit exceeds a specified value. That value is calculated as a function of the difference between a temperature determined just after the preceding defrost cycle during stable conditions before new frost has begun to form on the heat exchange unit and a predetermined minimum differential temperature.
The use of thermistor temperature sensors to determine coil and ambient temperature is also known. For example, U.S. Pat. No. 4,488,823, issued on Dec. 18, 1984 to D. A. Baker, describes a temperature control system for an air conditioner wherein a capacitor is charged, in sequence, through a reference resistor, a thermistor at ambient temperature, and a thermistor disposed on an evaporator coil. The time required for each to charge the capacitor is measured, and the charging times are used to calculate the ambient and evaporator coil temperatures. The compressor is inhibited when the evaporator coil falls below a predetermined temperature to prevent significant frost buildup.
Frost can be removed from an evaporator coil in a number of ways. For example, an electrical heating element in physical contact with the evaporator coil may be activated. Alternatively, the flow of coolant may be reversed, thereby reversing the roles of the evaporator and condenser coils.
In systems employing coolant flow reversing techniques for defrosting, however, the compressor must run during the defrost cycle. In such systems, if the compressor is switched on and off by a thermostat in a remote zone of temperature regulation, interruptions of the defrost cycle can occur. Additionally, accumulated "head" pressures in the compressor will be at a maximum when the compressor turns off. A substantial amount of energy is required to restart the compressor until the "head" pressure dissipates. To address this problem, a time delay relay may be employed to disable the compressor for a fixed period of time immediately following a compressor shutdown. A temperature control system for an air conditioner employing a timer (implemented in a microprocessor) to prevent compressor startup for a predetermined period after the compressor cycles off, is disclosed in the aforementioned U.S. Pat. No. 4,488,823 to Baker. However, in heat pump systems, discrete time delay relay units, operating independently of the remainder of the system, have typically been employed.